The invention is concerned with a process for the treatment of residues containing polluting and/or toxic materials. More specifically, the invention is directed to a process for treating residues containing oxidizable organic materials.
Today""s industries require practical solutions for the management of liquid and solid waste. For example, they must reduce the level of contamination of effluents before being released. Manuring and landfill of waste has become less popular, and incineration in large treating plants does not allow for the recovery of by-products. Liquid effluents and sludges generated by industries represent important concentrations of organic products as well as other oxidizable products. Each year, thousands of tons of organic materials and residues of various origins representing oxygen-demanding loads, are either treated or disposed of in landfills sites. However, an xe2x80x9con-sitexe2x80x9d treatment process represents an interesting alternative, especially if the process allows for the recovery of heat in the recycling of water and ashes produced.
The conventional approach, which comprises a drying step by evaporation, followed by an incineration step, is well known, but has limitations in terms of the energy yield. In particular, a lot of heat is lost from the air or the gases produced during the drying step. Also, many conventional incineration processes operate at high temperatures, for example around 1100xc2x0 C. The sludge to be incinerated must therefore be substantially dry and have a high concentration of combustible materials to obtain a self-heating process. Otherwise, external heating with gas burners will be required.
Oxidation at 1100xc2x0 C. requires a lot of energy to heat the gases and the water vapor. Therefore, incineration necessitates a heat source such as natural gas or fuel to initiate the reaction and maintenance thereof, as well as a high input of air that must be heated because the heat source already consumes a lot of oxygen. Furthermore, at such treatment temperature, salts and other volatile compounds escape from the incinerator. Also, the ashes, which contain alkali metal oxides, can form eutectics that are damageable to the refractory materials of the process or form melted heaps.
Landfilling of sludges requires large fields and is increasingly regulated because of long-term rigorous controls of liquid effluents. Landfill however has the advantage of being cheap and xe2x80x9capparentlyxe2x80x9d offers a simple and quick solution to the disposal of sludges. However, the waste producer always remain responsible for any damage to the environment resulting from such landfilling.
Manuring also has the advantage of being cheap, and often represent an alternative less expensive than landfill because it is done on the surface. Liquids as well as sludges with high concentration of organic materials can easily be manured. However, the materials must not be toxic, nor bearing pathogenic microorganisms, and must have a relatively neutral pH. Also, the leaching waters should not contaminate rivers, lakes or ground water. And even if the materials to be manured possesses fertilising properties, the available manuring sites should be as close as possible to the production site to avoid high shipping costs because of the presence of high water volume therein. Manuring nearby residential areas is generally unpopular because of the various odours that may emanate from the site.
Airy ponds cause problems in terms of the management of the sludges as well as the space required for the installations. Such process generally uses compressors to supply air under pressure by injection. A lot of electrical energy is used, but no energy is recovered or recycled.
Composting is applicable only to sludges containing biodegradable materials. Such sludges must be dry enough to avoid secondary leaching. If the sludge contains too much water, the liquid or the sludge can be mixed with straw to ensure good absorption of the liquids and good product aeration, but the downside is that the volume of product to be treated, the storage requirement and the length of treatment are significantly increased. Again, the product cannot contain toxic materials that would impair the effect of the composting microorganisms, and to ensure that the material produced can be released into the environment without causing damages thereto.
A few damp oxidations technologies in super critical phase or in liquid/gas phase are available commercially. One of the major drawbacks of the implementation of these technologies is the high investment required. This is caused by the size and complexity of the equipment necessary. The super critical oxidation process could be economically attractive, but is useful only for low effluent flows if the latter are highly toxic.
The present invention has been designed to overcome the above limitations.
The invention relates to a process for treating residues containing oxidizable organic matter. The process of the invention comprises the steps of:
a) introducing the residues in a rotating furnace having a refractory lining and containing a thermo-transfer agent;
b) heating the residues in admixture with the thermo-transfer agent at a temperature of at least 300xc2x0 C. in the presence of chemically active species and UV radiation generated electrically, while rotating the furnace, so as to dry the residues and cause oxidation of the organic matter and formation of gases, the organic matter oxidation being catalysed by the chemically active species and the UV radiation; and
c) expelling the gases from the furnace.
The process of the present invention allows the drying and destruction by oxidation of the organic matter contained in the residues treated. It is based on the use of an electrically catalysed oxidation reactor, in particular for the treatment of effluents comprising high organic matter concentration, such as those found in the pulp and paper, textiles, paper de-inking, and food industries. Electricity is used to induce catalytic effects and transport phenomenons in combustion reactors.
Preferably, step b) is performed with a plasma torch. It is also possible to use electric arc system or an electrical discharge system comprised between two electrodes made of metals or graphite. These devices allow the heating of the mixture of the residues and the thermo-transfer agent at the desired temperature, and permit a generation of an ultraviolet-type radiation as well as chemically active species, such as free radicals, ions and excited species, which catalyzed the oxidation reactions. This is a rational use of electricity for the generation of phenomenons assisting transport mechanisms in chemical reactions taking place in an oxidation reactor. As a result, incineration process is performed in a smaller reactor in comparison with conventional incineration processes.
The process according to the invention aims at taking advantage of the calorific value of the oxidizable matter contained in the residues to be treated, to help the water evaporation process in the thermal destruction of the polluting materials. For sufficiently concentrated sludges (dryness higher than 10%), and if the calorific power of the oxidisable materials is sufficiently high, it will be possible to reach a self heating operation, and even exothermic, for the oxidation reactor. The expression xe2x80x9cdrynessxe2x80x9d is defined as the mass proportion including the organic matter and the inorganic matter, excluding water, with respect to the total mass of the damp residues for diluted liquid effluents. The concentration of these matters can be increased through conventional standard techniques. These include the use of separation processes with membranes, such as inverted osmosis, ultrafiltration, nanofiltration and/or microfiltration, the latter being tangential flow filtration processes. Also included are the mechanical dehydration processes with or without flocculation/coagulation products, such as centrifuges, screw presses, seave filters, press filters, sedimentation processes, and the like. These dehydration processes may or may not be used in conjunction with separation processes using membranes as outlined above. The present process can be used for directly incinerating sludges obtained from a dehydration process, as well as concentrated sludges produced from membrane separation processes.
The oxidation is not affected by the pH of the residues or the presence of pathogenic microorganisms. The decomposition of toxic compounds like cyanide or ammonia is not problematic. Furthermore, the present process produces ashes that are inorganic, inert and sterile that may represent worthy by-products. In the treatment of organic sludges of biological origin, the ashes produced can be used in fertilisers compositions because they may contain nitrates, phosphates, and other inorganic compounds.
It is also possible to destroy polyhalogenated hydrocarbons with fixation of halogens because of the high local temperatures available through the electrical plasma and the presence of alkaline ashes in the solids. Additives may be added in the firnace during the treatment, and is recommended to fix toxic products. Halogen fixation is generally accomplished by adding oxides, hydroxides or carbonates of alkaline metals or akaline-earth metals. Fixation of heavy metals is accomplished by adding phosphoric acid or phosphates and/or carbonates of alkaline metals or alkaline-earth metals, while hexavalent chromium is fixed by adding ferrous sulphates.
In the preferred embodiments of the invention, the temperature of the side walls of the furnace is maintained between 300 and 900xc2x0 C. during step b), and preferably at about 500xc2x0 C. A minimum temperature of 300xc2x0 C. is required to ensure a spontaneous oxidation reaction of the organic matter contained in the residues. At 500xc2x0 C., the solids do not melt, there is no fritting, nor eutectic formation with the refractory materials. However, some fine solids may agglomerate because of the mechanical action of the rotation of the furnace, which is desirable to avoid the evacuation of dust through the gases formed during step b), and ensures good fluidity of the thermo-transfer agent.
Step b) is generally performed at ambient pressure, which is about 100 kPa. It is however possible to operate at pressures varying from 30 to 600 kPa.
Preferably, the furnace has a size and geometry creating an internal volume allowing residence time of the gases of at least 1 second so that the oxidation reaction is complete inside the furnace. If the residues contain inorganic matter, the residence time of the residual inorganic matter (ashes) is generally higher than that of the gases so that adequate heat transfer is provided between the ashes generated and the residues freshly introduced, and to allow the ashes to accumulate inside the furnace to form the thermo-transfer agent. The filling level of the coolant in the furnace preferably corresponds to 15% of the total internal volume.
The term xe2x80x9cthermo-transfer agentxe2x80x9d as used herein refers to a solid and granulous inorganic material comprising particles having a diameter greater than 10 xcexcm. It is used to make the heat of the refractory lining of the rotary furnace uniform through contact between the particles and the lining. It also quickly transfers the energy of the furnace to the humid residues introduced in the furnace, by admixture therewith. The thermo-transfer agent comes from the inorganic material contained in the residues to be treated which are deposited as ashes, and agglomerates, and/or can be introduced in the furnace if the residues do not contain inorganic matter or how a low concentration thereof. Examples of thermo-transfer agents that can be introduced in the furnace comprise sand, alumina particles, ground ceramics, etc.
Preferably, a furnace having a treatment capacity of at least 50 kg/h and a tangential speed of rotation of at least 0.01 m/s at the inner side wall of the furnace, but inferior to the speed of centrifugation of the furnace. Such a tangential speed of rotation allows the fluidization of the thermo-transfer agent, its mixture with freshly introduced residues and heat transfer between the refractory lining and the thermo-transfer agent.
The process of the invention is particularly useful for treating humid residues having a dryness or dry matter content comprised between 10 and 65% by weight, based on the total weight of the humid residues, and containing up to 70% by weight of organic matter, based on the total weight of dry organic and inorganic matters. The use of such residues makes possible its treatment in self-heating conditions.
Humid residues having a dryness higher than 35% by weight can be treated conventionally in an incinerator. Humid residues having a dry matter content lower than 10% by weight are too diluted and would be better treated by either a supercritical oxidation process, ozonation or other technologies for treating diluted aqueous effluents. At concentration of inorganic matters higher than 70% on a dry basis, the calorific power of the dry matter is too weak, and the dry matter content must therefore be significantly higher than 65% to insure self-heating conditions.
The process of the invention is useful for treating humid residues such as:
a) waters highly toxic or containing a high concentration of pollutants, originating from industrial processes such as de-inking, textile, tanin, chemical industries, pulp and paper (secondary sludges and black liquor), etc.;
b) sludges and liquid effluents generated in agro-food industry, such as animal excrements, effluents containing fats proteins, glucids, etc.;
c) sludges from clarifyers used in wastewater treatment;
d) sludges from biological treatment systems; and
e) vegetable residues with a high water content, such as plant pulps like corn, soya and others.